With the increased awareness of environmental issues such as global warming it is very desirable to be able to recycle plastic materials, in particular those used in packaging. One such source of plastic materials is known as Post Consumer Recycled (PCR). In general, 3 types of polymer are available as PCR, these being polyethylene terephthalate (PET), polypropylene (PP) and polyethylene (PE). The quality of these materials has made significant progress and PET, the most commonly used in packaging, has reached the status of food grade quality and is often recycled into bottles. PE and PP have not known the same level of development and only more recently have started to find applications into packaging. Hence, simply melting and reforming PP or PE from a PCR source often results in compromised properties and defective products. Specifically, reusing high density polyethylene container scrap (HDPE) in anything other than very low quantities provides containers having diminished physical properties, in particular diminished resistance to stress cracking.
Stress cracking is a phenomenon known to packaging such as containers and can be evidenced by the appearance of hazy cracks in the container which can be, at a minimum, unappealing to the consumer or at their worst result in leakage from the container.
Stress cracking is a property associated with long-term behavior: when the material under stress is immersed into a surfactant liquid, a crack will appear after a certain time. This phenomenon, which is very important in the case of polyethylene, is highly dependent on the surface tension exerted by the medium and the morphology of the polymer. It is known that the crack occurs sooner when the stress is higher and the material (polyethylene in this case) has a high melt index.
To avoid stress cracking, the materials used to make the container are carefully selected, with experience showing that in the field of PE materials lower density grades perform better that high density grades. For this reason accelerated testing methods have been developed over the years to allow packaging engineers to evaluate the long term risk of stress crack appearance.
A growing source of PCR material that could potentially be used to make containers comes from bottles used initially for storing milk and juices. However, this material is traditionally a HDPE grade of higher density than that conventionally used for some containers, such as tubes for instance. PCR containers made with a high PCR HDPE content hence have a higher density than containers made with virgin HDPE for these applications. It is believed that this higher density contributes to the observed poor Environmental Stress Crack Resistance (ESCR) results. Thus, very few applications involving containers for consumer products (such as filled tubes) use significant quantities of PCR HDPE.
Despite these inherent difficulties, the growing quantities of post consumer HDPE, the increased level of colour sorting and decontamination (with the arrival of food grade quality), it is very desirable to use such PCR HDPE. The preferred process to make such tubes would be to simply replace all or some of the virgin HDPE by quantities of PCR HDPE and extrude the material as a monolayer blend. However, such monolayer extruded blends of PCR HDPE do not have adequate ESCR properties for market use. ESCR properties are evaluated using accelerated testing protocols and can be defined as the number of days elapsed before observing the first failure of a container or a batch of containers subjected to ESCR test conditions. For the field of the present invention, adequate ESCR properties refer to at least 6 days elapsed before a first failure of a container during an ESCR test.
One prior art solution co-extrudes the PCR HDPE with one or more other resins so that the PCR layer is an external (surface) layer of a container (hence isolated from the inner layer in contact with liquids) or the PCR HDPE is encapsulated between virgin layers. However, co-extrusion is expensive, and thus it is desired to avoid coextrusion.
Another solution is offered by U.S. Pat. No. 5,783,637 wherein PCR HDPE is fusion blended with virgin HDPE and LLDPE to reduce or eliminate the ESCR problem. Fusion blending or compounding is an extra step in which the PCR HDPE and a modifier are mixed/compounded together in a different extruder, prior to the actual extrusion step to form the container, in order to form pellets; these pellets are then fed into the extruder and mixed with the virgin HDPE. This additional fusion blending operation would be expected to improve the mixing of the two materials to form a more homogenous blend, and hence improve the ESCR performance. However, this extra step requires additional cost and preparation time.
However, even when incorporated as a blend with virgin polyethylene (PE) resins, the higher the HDPE PCR content, the worse the ESCR properties of the blended resins. That is, ESCR properties are reduced with increased PCR HDPE content in blends, especially when greater than 50% PCR HDPE is desired, to achieve maximal recycled content. In the solution described in U.S. Pat. No. 5,783,637 the number of days elapsed to the first failure is always smaller than 6, for blends including a PCR content of at least 50%.
A solution is desired which allows an increased percentage of PCR HDPE to be used, having adequate ESCR properties, without requiring the use of virgin HDPE.